3815_PGSF WMVD Pkg 2_Vol 4-2_03.03.2026_BXWA_Certified

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 75 greater than 10 that are shaded in blue. Despite the Site-wide average declines in groundwater pH, Figure 6-P8 also illustrates that high-pH groundwater persisted into the fall of 2020, 7 years after placement of the CM, in areas where CM was seasonally or perennially in contact with groundwater. 6.3.1.1 pH Trends at Inland High-pH Wells Figure 6-P11 depicts the measured groundwater pH trends between 2016 and (post-CM removal) 2021 for high-pH wells in areas where CM occurred beneath the water table either seasonally or perennially (wells GF9-MW-2, PM-MW-4, TM-MW-3, TM-MW-4, and UST29-MW-101). The trend for high-pH well LP-MW-1 is shown on Figure 6-P10. so is not repeated on Figure 6-P11. Each of the five high-pH wells on Figure 6-P11 had pH measurements collected in 2016, 2017, 2020, and 2021 except for TM-MW-3 and TM-MW-4, in the northeast portion of the Site, which were decommissioned prior to the start of the CM removal project. The post-CM-removal groundwater pH measurements were collected by Landau Associates from exploratory test pits in June 2021. The test pits were generally positioned adjacent to decommissioned monitoring wells where the highest groundwater pH had been measured in prior monitoring events to assist with siting of locations to temporarily infiltrate stormwater during construction of Port’s third IA. Figure 6-P9 shows the test pit locations and pH measurements with the prior monitoring wells for reference. The 2016-2017 data show substantial seasonal variability as described above, with generally lower pH in 2020 and then considerably lower pH in June 2021, roughly 8 months after completion of CM removal. As shown on Figure 6-P9, the pH measured in test pit LA-TP-4, located next to former high-pH well LP-MW-1, was at pH 7.8. As of the June 2021 measurements, only the central inland area adjacent to former well GF9-MW-2 remains above pH 8.5 (Figure 6-P9). 6.3.1.2 pH Trends at Shoreline Wells Despite the widespread groundwater pH increases where CM was placed across the interior of the Upland Area, the data indicate that high-pH groundwater did not migrate to the groundwater discharge zone at the shoreline, with one localized short-term exception. Figure 6-P12 illustrates groundwater pH trends measured at wells across the entire Upland Area shoreline from 2012, prior to CM placement, through 2020 when the CM removal was occurring (the 2020 data points are the average of measurements collected over multiple days). The shoreline groundwater data show temporal variability but no clear trends over the 8-year period of monitoring. Exceptions are the measurements greater than pH 9.0 collected in February 2016 from four wells in Site Unit B (PM-MW-8, REC3-MW-1R, UST70-MW-102, and UST70-MW-2). Figure 6-P3 shows the locations of these wells and the February 2016 data. In May 2016, the pH readings were 8.9 at the two UST70 wells and were 7.2 or below at the other two wells. By August 2016, those four wells were below pH 8.0 and they remained below pH 8.0 thereafter (Figure 6-P12; Table 6-11).